This invention relates to a ballast circuit for a gaseous discharge lamp. More particularly, this invention relates to such a circuit for generating special decorative effects in the illumination or energization of gaseous discharge lamps such as neon lights.
The power supplies and electronic drive circuitry that are currently being used to operate high-voltage neon tubes genrally consist of a high-voltage transformer and high-power solid state electrical devices. These power units are frequently very bulky, costly and inefficient, particularly if any type of display variations are incorporated into the design. Moreover, the high voltages present in the devices result in a potentially dangerous situation if a tube is accidentally damaged or broken.
With respect to possible display variations, it is known to control the energization of a neon light to produce a "writing" effect wherein the illuminated portion of the neon tube gradually increases in length from one end of the tube towards the other end thereof. Other special decorative effects which are achievable in neon lighting include flashing or blinking, a "bubbling" or striation effect, and a dimming or light modulation effect.
U.S. Pat. No. 3,440,488 to Skirvin discloses a circuit connectable to a gas-filled luminescent tube for illuminating progressive portions of the tube, i.e., for achieving a "writing" effect. The increase in the length of the illuminated portion of a gas-filled luminescent tube is achieved by varying the voltage, current and/or frequency of the input excitation signal. A resonant tank circuit including a capacitor and the primary winding of a transformer operatively connected to the flourescent tube is fed a polarized waveform having a frequency which is increased as power input to the waveform generating circuit (comprising a silicon controlled rectifier) is increased. The power supplied to the waveform generating circuit is increased by increasing the "on" time of another silicon controlled rectifier via a light source and a light sensitive potentiometer.
U.S. Pat. No. 4,682,082 to MacAskill et al. relates to an electronic energization circuit for illuminating a gas discharge lamp and includes a transformer with a rectangular hysteresis loop. A secondary winding of the transformer is connected to the gas discharge lamp, while at least one primary winding of the transformer is connected to a transistor in turn tied to input terminals of the energization circuit. The transistor is controlled to have unequal on and off periods to eliminate striations in the gas plasma of the discharge lamp.
U.S. Pat. No. 4,415,839 to Lesea describes and illustrates an electronic ballast circuit with two series connected MOSFETs having a common output connected to a gaseous discharge lamp via a voltage-conditioning and current-limiting network. The MOSFETs are also connected to a d-c power supply and to a pulse generating circuit which turns the MOSFETs alternately on and off in response to feedback signals from the load and from a source terminal of one of the MOSFETs. In one embodiment of the ballast circuit, the signal on the common output of the two MOSFETs is a series of alternating positive and negative pulses varying in frequency and duration. In another embodiment of the ballast circuit, the load is driven by a triangular wave signal which is amplitude modulated in response to a feedback signal.
U.S. Pat. No. 4,087,722 to Hancock involves a circuit for energizing a gaseous discharge tube. The energization circuit includes a subcircuit for generating a square wave signal of varying pulse width to vary the output intensity of the gaseous discharge tube in accordance with ambient light conditions. The subcircuit is provided with photoresistors which change their resistance in response to the ambient light and thereby alter the trigger times of a pair of silicon controlled rectifiers. A flashing effect in the gaseous discharge tube is implemented by a transistor which grounds trigger inputs of the silicon controlled rectifiers under the control of a timing circuit.
U.S. Pat. No. 4,704,563 to Hussey discloses a fluorescent lamp operating circuit with a system for intensity control. At the center of the intensity control system are two transistors connected in a half-bridge arrangement and switched by high frequency signals produced by a pulse width modulation controller. The controller is triggered if two successive binary codes are detected on a power line by a receiver circuit. The output of the two transistors is fed to the primary winding of a transformer.
U.S. Pat. No. 4,492,899 to Martin is directed to a solid state regulated power supply for a cold cathode luminous tube, wherein the repetition rate of power pulses to the luminous tube is varied to compensate for temperature and load changes. The tube is connected to a secondary winding of a transformer having a primary winding connected on one side to a power source and on an opposite side to a transistor switch. The frequency of a control signal fed to the base of the transistor changes in response to variations in a feedback signal originating at an auxiliary secondary winding of the transformer. The power supply includes several potentiometers for setting power, pulse width and temperature zeros or norms.